Epidermal cytokines in contact hypersensitivity: Immunological roles and practical applications

The Role of Structure-activity Relationships and Expert Systems in Alternative Strategies for the Determination of Skin Sensitisation, Skin Corrosivity and Eye Irritation

A historical database containing the results of 294 defined single substances tested in the guinea-pig maximisation test, carried out according to a single protocol, was used to derive a set of structural alerts for skin sensitisation, which have been incorporated into the expert system, DEREK. Together with an assessment of percutaneous absorption, this system forms an integral part of a strategic approach to the identification of contact allergens.

Quantitative structure-activity relationships (QSARs) were derived for the skin corrosivity of organic acids and bases, and for the eye irritation potential of neutral organic chemicals. The independent variables used for these analyses were selected on the basis of the putative mechanisms for skin irritation or corrosivity and for eye irritation, respectively. Data sets were analysed using principal components analysis; plots of the first two principal components for each data set showed that the analyses were able to discriminate well between chemicals with different classifications of toxicological activity. The derived QSARs are expected to give useful predictions of skin corrosivity and eye irritancy for new or untested chemicals in these classes.

Although the development of these techniques is still at a very early stage, they are already able to play an important part in proposed strategies for the reduction of experimental animal usage. In the long term, it should be possible to conduct safety evaluations using fewer experimental animals or no animals at all. However, acceptance by regulatory authorities will be a key factor in realising the full benefits of the approach.

Structure-activity relationships for selected fragrance allergens

Fragrance substances represent a very diverse group of chemicals, a proportion of them providing not only desirable aroma characteristics, but also being associated with adverse effects, notably the ability to cause allergic reactions in the skin. However, efforts to find substitute materials are hampered by the need to undertake animal testing to evaluate both the presence and the degree of skin sensitization hazard. One potential route to avoid such testing is to understand the relationships between chemical structure and skin sensitization. In the present work we have evaluated two groups of fragrance chemicals, saturated aldehydes (aryl substituted and aliphatic aldehydes) and alpha,beta-unsaturated aldehydes. Data on their skin sensitization potency defined using the local lymph node assay has been evaluated in relation to their physicochemical properties. The initial outcome has been consistent with the concept that alpha,beta-unsaturated aldehydes react largely via Michael addition, whilst the group of saturated aldehydes form Schiff bases with proteins. Simple models of chemical reactivity based on these mechanisms suggest that it may be possible to predict allergenic potency. Accordingly, the evaluation of an additional group of similar aldehydes is now underway to assess the robustness of these models, with some emphasis being based on ensuring a wider spread of chemical reactivity.

Redox-modulated pathways in inflammatory skin diseases

Inflammatory skin diseases account for a large proportion of all skin disorders and constitute a major health problem worldwide. Contact dermatitis, atopic dermatitis, and psoriasis represent the most prevalent inflammatory skin disorders and share a common efferent T-lymphocyte mediated response. Oxidative stress and inflammation have recently been linked to cutaneous damage in T-lymphocyte mediated skin diseases, particularly in contact dermatitis. Insights into the pathophysiology responsible for contact dermatitis can be used to better understand the mechanism of other T-lymphocyte mediated inflammatory skin diseases, and may help to develop novel therapeutic approaches. This review focuses on redox sensitive events in the inflammatory scenario of contact dermatitis, which comprise for example, several kinases, transcription factors, cytokines, adhesion molecules, dendritic cell surface markers, the T-lymphocyte receptor, and the cutaneous lymphocyte-associated antigen (CLA). In vitro and animal studies clearly point to a central role of several distinct but interconnected redox-sensitive pathways in the pathogenesis of contact dermatitis. However, clinical evidence that modulation of the skin's redox state can be used therapeutically to modulate the inflammatory response in contact dermatitis is presently not convincing. The rational for this discrepancy seems to be multi-faceted and complex and will be discussed.

Further evaluation of the local lymph node assay in the final phase of an international collaborative trial

The local lymph node assay (LLNA) is a method used for the prospective identification in mice of chemicals that have the potential to cause skin sensitization. We report here the results of the second and final phase of an international trial in which the performance of the assay has been evaluated using seven test materials in five independent laboratories. The additional chemicals examined here included compounds which are considered less potent allergens than some of those tested in the first phase of the investigation, and includes hexylcinnamic aldehyde (HCA), a chemical recommended by the Organization for Economic Cooperation and Development (OECD) as a positive control for skin sensitization studies. In each laboratory all skin sensitizing chemicals examined (2,4-dinitrochlorobenzene {DNCB}, HCA, oxazolone, isoeugenal and eugenol) elicited positive responses of comparable magnitude as judged by the derived lowest concentration of test chemical required to elicit a 3-fold or greater increase in the proliferative activity of draining lymph node cells compared with vehicle-treated controls. We observed that sodium lauryl sulphate, considered to be a non-sensitizing skin irritant, also induced a positive response in the assay. Para-aminobenzoic acid (pABA), a nonsensitizing chemical, was negative at all test concentrations in each laboratory. Some laboratories incorporated minor modifications into the standard assay procedure, including the evaluation of lymph nodes pooled from individual mice rather than treatment groups and the use of statistical analyses. The use of statistics did not markedly change the determination of the lowest concentration yielding a positive response. These data confirm that the local lymph node assay is robust and yields equivalent results when performed independently.

Stimulation by oxazolone of increased IL-6, but not IL-10, in the skin of mice

Interleukin 6 (IL-6), a multi-functional cytokine, is expressed constitutively by epidermal Langerhans cells and can be produced also by keratinocytes following stimulation. We have investigated the production of cutaneous IL-6 following topical exposure of mice to oxazolone, a potent contact allergen. Homogenates of ear skin prepared from untreated BALB/c strain mice, or from mice exposed on the dorsum of both ears to vehicle alone, contained low levels of IL-6. Topical application of oxazolone to the ears of mice induced a rapid and dose-dependent increase in IL-6 expression that was maximal 4-8 h following application and remained elevated for up to 24 h. Under the same conditions of exposure the expression of a second epidermal cytokine, interleukin 10, a product of keratinocytes, was unaffected. It is concluded that topical exposure to the chemical allergen oxazolone results in the selective stimulation of cutaneous cytokine production. The experimental approach described herein provides a simple and reliable method for the investigation of induced changes in the expression of skin cytokines.

Cytokine regulation of chemical sensitization

The skin is an immunologically active tissue. Epidermal cells, both keratinocytes and Langerhans cells (LC), produce constitutively or can be stimulated to produce a variety of cytokines, many of which play important roles in the induction and regulation of allergic responses to sensitizing chemicals. Tumor necrosis factor alpha (TNF-alpha) provides the signal for LC migration from the skin and granulocyte/macrophage colony-stimulating factor (GM-CSF), interleukin 1 (IL-1) and other cytokines effect the functional maturation of LC and their acquisition of immunostimulatory potential. The initial stimulus for induced or increased epidermal cytokine production derives from chemical exposure, or some other form of skin trauma. However, some epidermal cytokines are regulated in paracrine or autocrine fashion by other cytokines produced locally. The availability of epidermal cytokines has a major impact on the induction of sensitization and on the characteristics of immune responses to chemical allergens.

Retrospective appraisal of the relationship between skin irritancy and contact sensitization potential

A retrospective analysis of the association between skin irritancy and the potential to cause contact sensitization has been performed employing a historical database for 50 chemicals and formulations. Correlations between the results of Draize skin irritation tests and skin sensitizing activity measured with the occluded patch test of Buehler have been examined. Weak, but nevertheless statistically significant, associations between contact sensitization and skin irritancy have been demonstrated. It is proposed that such correlations are consistent with the irritant properties of a material exerting an important influence on the extent to which contact sensitization is induced.

Cytokines and adherence molecules involved in spontaneous dendritic cell-lymphocyte clustering in skin afferent lymph

The skin afferent lymph dendritic cell (DC) spontaneously forms clusters with autologous T cells. The role of adhesion molecules and cytokines in this process was investigated. Analysis of the expression of adhesion receptors on the canine peripheral lymph DC revealed the presence of CD54, CD58, CD18 as well as CD49d and CD49e molecules and cell surface fibronectin. The CD54 and CD58 molecules were found to play a key role in the 'spontaneous' lymph cell clustering. Antibody against fibronectin, a substrate for CD49d and CD49e receptors, reduced DC-lymphocyte binding. Analysis of the effect of cytokines revealed that the pro-inflammatory IL1 beta rather than IL1 alpha, and TNF alpha may be responsible for the enhanced lymph cell in vitro clustering. The IL6 had no such augmenting effect. The enhancing effect of endogenous IL1 beta present in lymph was reduced by the IL1 beta neutralizing antibody. The effect of exogenously added IL1 beta was also limited by the IL1 receptor antagonist. The IL1Ra alone had no effect on cell binding, even when used in the high doses. Neutralizing of IL1Ra in lymph with the specific antibody brought about augmented cluster formation. The enhancing properties of TNF alpha on cell binding were reduced by the TNF alpha neutralizing antibody. The IL10 significantly limited lymph DC cluster formation with T cells. In conclusion, these data demonstrate that the present in lymph IL1 beta and TNF alpha may be responsible for the observed in vitro enhanced cluster formation of lymph DC with autologous T lymphocytes. Cell binding can be reduced by IL1Ra and by IL10. It provides insight into the potential clinical use of these inhibitors.